Operation Cannot consume content type is not supported. when trying to upload a file to sandbox-quickbooks.api.intuit.com - quickbooks

Question

We are making a POST to
sandbox-quickbooks.api.intuit.com/v3/company/{realm_id}/attachable
with header:
Content-Type: multipart/form-data; boundary="--fdlkjflksd--"
and body:
--fdlkjflksd--
Content-Disposition: form-data; name="file_metadata_01"; filename="attachment.json"
Content-Type: application/json; charset=UTF-8
Content-Transfer-Encoding: 8bit
{"AttachableRef": [{"EntityRef": {"type": "Purchase", "value": "144"}}], "FileName": "one_pixel.jpeg", "ContentType": "image/jpeg"}
--fdlkjflksd--
Content-Disposition: form-data; name="file_content_01"; filename="one_pixel.jpeg"
Content-Type: image/jpeg
Content-Transfer-Encoding: base64
/9j/2wBDAP//////////////////////////////////////////////////////////////////////////////////////wAALCAABAAEBAREA/8QAFAABAAAAAAAAAAAAAAAAAAAAA//EABQQAQAAAAAAAAAAAAAAAAAAAAD/2gAIAQEAAD8AN//Z
--fdlkjflksd--
and we are getting in response:
{"Fault":{"Error":[{"Message":"Unsupported Operation","Detail":"Operation Cannot consume content type is not supported.","code":"500"}],"type":"ValidationFault"},"time":"2020-03-07T03:00:51.600-08:00"}
we followed this example

Answer 1

The /attachable endpoint is for linking EXISTING attachments to an object, and the Content-Type is:
application/json
Docs are here: https://developer.intuit.com/app/developer/qbo/docs/develop/tutorials/attach-images-and-notes#attaching-a-note-to-an-object
The /upload endpoint is for uploading NEW attachments, and the Content-Type is:
multipart/form-data
Docs: https://developer.intuit.com/app/developer/qbo/docs/develop/tutorials/attach-images-and-notes#uploading-files-to-the-attachment-list
It looks like you've mixed then up - it looks like you're trying to upload a new file to the existing file endpoint.